1. Field of Invention
The invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus applies a desired pattern onto a target portion of a substrate. Lithographic apparatus are used, for example, in the manufacture of integrated circuits (ICs). When so used, a patterning device, such as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be imaged onto a target portion (e.g. corresponding to a portion of one more dies) of a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in a single operation, and so-called scanners, in which each target portion is irradiated by scanning the pattern using a beam moving in a given “scanning”-direction with respect to the substrate while synchronously scanning the substrate parallel or anti parallel to this direction.
Lithographic apparatus include a source configured to generate radiation which is used to illuminate the patterning device, and thereby allow a pattern present on the patterning device to be projected onto the substrate. The source may for example be a laser which is configured to emit ultraviolet radiation (e.g. radiation having a wavelength of 240 nm, 193 nm or some other wavelength). Radiation emitted by the laser may have a spectrum which is spread over for example 0.3 picometers (pm), and in some instances may have a spectrum which is spread over for example 1 to 2 pm. The spectrum may include several peaks.
It is desirable to provide improved control of the spectrum of laser radiation emitted by a laser radiation source, thereby allowing a pattern to be projected more accurately onto a substrate by a lithographic apparatus.